Pressure gradient responsive microphone



Oct. 24, 1950 J, ANDERSON 2,527,344

PRESSURE GRADIENT RESPONSIVE MICROPHONE Filed Jan. 30, 1947 7 1/ w/r/r i g I amsuei x Z5 E T--\ 3 (IV/T7007) 21 I I 1 4mm: 5F 19 I I a l a. /a0 moo M000 7 Z5 reip, 0 645 if, 77 l?- 11 A INVENTOR ATTORN EY Patented Oct. 24, 1950 PRESSURE GRADIENT RESPONSIVE MICROPHONE Leslie J. AndersomMoorestow n, N. J assignor to Radio Corporation of America, a corporation of Delaware Application January 30, 1947, Serial No. 725,334

13 Claims. (01. 179-1155) This invention relates to microphones, and

more particularly to a microphone of the pressure gradient responsive type, the present invention being in the nature of an improvement on that disclosed in Olson Patent No. 1,885,001.

In the aforesaid patent, there is disclosed a microphone which consists of a vibratile, electroconductive element disposed in the air gap of a magnetic circuit and so arranged that both sides thereof are open to the ambient in which the microphone is immersed, the vibratile element having associated therewith a baffle around which the sound waves must pass in going from one side of the element to the other. The difference in sound pressure due to a difference in phase between the pressures on the two sides of the vibratile element is the source of the force which drives the element.

One difficulty with a microphone of this type is that when the path length from the front surface to the back surface of the vibratile element approaches a value close to one-fourth of the wave length at the operating frequency, the frequency response curve shows a considerable drop.

The primary object of my present invention is to provide a pressure gradient responsive microphone which will not have this disadvantage and which will have a more uniform response over the operating range.

More particularly, it is an object of my present invention to provide an improved pressure gradient responsive microphone in which the high frequency response is greatly enhanced in comparison with that of conventional microphones of this type.

Another object of my present invention is to provide an improved pressure gradient responsive microphone the high frequency response of which can be adjusted to obtain more nearly uniform response over the entire operating range of the microphone.

Still another object of my present invention is to provide an improved pressure gradient responsive microphone which can be readily adjusted to accentuate or enhance the response at one or more preselected frequencies, particularly within the high frequency range.

It is also an object of my invention to provide an improved pressure gradient responsive microphone as aforesaid which is very simple in construction, economical in cost, and highly efficient in use.

In accordance with my present invention, one or more closure members may be disposed opposite one side of the vibratile element or conductor of the microphone, each of these closure members being open at its ends and formed to provide a cavity which is resonant at substantially one or more preselected frequencies at which the microphone response is to be enhanced. The closure members may be constituted by solid blocks in which suitable cavities have been formed, or they may be made simply of pipe or tubing. These members are arranged in longitudinally spaced relation along the length of the vibratile conductor, each one being of a length which is of the order of a half wave length at the frequency at which maximum compensation is desired. In considering the length of each of these closure members, cognizance must be taken of the end correction. The amount of compensation and the relative sharpness of the response may be adjusted by changing the ratio of the area of the cavity to the length of the closure member, the smaller the cross sectional area of the cavity, the greater the compensation.

The novel features of my invention, as well as additional objects and advantages thereof, will be understood better from the following description of several embodiments thereof, when read in connection with the accompanying drawing in which Figure 1 is a rear perspective view of one form of microphoneaccording to my present invention,

Figure 2 is a sectional view taken along the line IIII of Figure 1, c

Figure 3 is a longitudinal, sectional view taken along the line IIL-III of Figure 2,

Figure 4 is a graph showing the response 'of my improved microphone in comparison with the response of conventional microphones of the same type,

Figure 5 is a fragmentary, sectional view similar to Figure 2 but showing a slightly modified form of microphone according to my present invention, and

- by the magnet 3 in known manner. Vibratively by the dash lines A in Figure 3.

reach the adjacent surface of the ribbon I.

mounted in the air gap is an elongated, conductive, crimped ribbon I of well-known form. The longitudinal or side edges of the ribbon I are spaced somewhat from the adjacent edges of the pole pieces I so as to provide slits 5a which afford adequate clearance to permit the ribbon I to vibrate freely in response to sound waves impinging thereon. I

Secured to the pole pieces I, as by screws 9, are one or more closure members I I which are disposed opposite one side or surface of the ribbon I. In the particular forms of the invention illustrated in the drawing, the members II are constituted by sections of pipe or tubing formed with flanges I3 having openings throughwhich the screws 9 pass. Each of the closure members H is open at both its endsfas can be seen best from Figure 3, and each one is of a length which is of the order of a half wave length at the frequency at which the maximum high frequency compensation is desired. The closure members -II thus provide cavities I5 opposite the conductive ribbon element 1 along the length'of which a half wave length standing wave is set up at the frequency at which maximum compensation is desired. These standing waves are represented Thus, in the region of the preselected frequency at which maximum compensation is desired, the high frequency response of themicrophone will be great- 1y enhanced, and the microphone will show a considerable improvement in the overall response over similar microphones of the prior art. The improvement in response can be seen from the curves of Figure 4 in which the curve B represents the response of a conventional pressure gradient responsive microphone, and the curve C represents the response of my improved microphone.

Where a single closure member I I is employed, it may be placed opposite the ribbon I near its center or at any other suitable point. Where two or more such closure members are employed,

these are spaced longitudinally from each other along the ribbon I, as shown in Figure 3, and preferably at least one of them is disposed in proximity to one (the upper) end of the ribbon I. The cavities I5 may or may not all be resonant at the same frequency, but in any case, the length of any pipe section or other closure member should be such' that, when its end 001'- rection is taken into account, it will have a length which is substantially equal to one-half the wave length at the frequency at which its cavity I5 is to be made resonant.

In operation, sound from a given source will be free to pass down through the open ends of the members II and through the cavities i5 which communicate with these open ends to At the same time, the sound will be free to also reach the opposite surface of the ribbon I. The difference in pressure On the ribbon at its two surfaces will provide the driving force necessary to vibrate the ribbon. At the frequencies at which the cavities I5 are resonant, the pressure gradient on the two surfaces of the ribbon I will be increased over that found in conventional pressure gradient responsive microphones, and

increased output will be obtained.

As stated previously, the amount of compen- 'sation and the relative sharpness of the increase in response may beadjusted by changing the ratio-of the cross-sectional area of the cavity M5 to the lengthof its associated member II.

This change in ratio may be accomplished, for example, by removin any given members I I and replacing them with similar members of greater or smaller diameter, as may be required. Where, for example, the members I I are permanently secured to the pole pieces I, as by soldering, the area of the cavity I5 may bechanged by inserting a filler I'I thereinto, as shown in Figure 5.

In some cases, it may be found desirable to alter the length of the closure member II. For this purpose, a construction such as shown in Figure 6 may be employed. In this form of my invention, the closure member I I is provided with a slot I9 through which extends a threaded stem III on a sectionof pipe or the like 23 which is slidably received in the member II. The pipe section 23 may be adjusted along the length of the member I I and locked in place in any desired position by means of a nut 25 threaded on the stem 2|. In this way, the effective length of the closure member maybe altered to .thereby shown in the accompanying drawing shall be tak-' on as illustrative and not as limiting.

I claim as my invention:

1. In a pressure gradient responsive microphone, the combination of means providing. a magnetic path including an air gap, a conductor mounted for movement in said air gap in response to differences in sound wave pressure on opposite sides thereof, and at least one closure member disposed opposite one side of said-conductor over a short length thereof, said closure member providing a correspondingly short cavity opposite said conductor side which is resonant at substantially a preselected frequency whereby to enhance the response of the microphone inthe region of said frequency and being open at both its ends to permit access of sound waves to said conductor at its said side.

2. A pressure gradient responsive microphone according to claim 1 wherein said member is so arranged that its said open ends are spaced from each other longitudinally along said conductor.

3. A pressure gradient responsive'microphone according to' claim 1 wherein said member is so arranged that said cavity extends longitudinally along said conductor and terminates in-said' open ends.

4. In a pressure gradient responsivamicrophone, the combination'of means providing a magnetic path including an air gap, a conductor mounted for movement in said airgap: in-response to differences in sound wave pressure on opposite sides thereof, and means for enhancing the high frequency response of the microphone, said means comprising at least one closure member disposed opposite one side of saidconductor over only a short length thereof and providing a correspondingly short cavity'oppositesaid conductor side which is resonant at substantially a preselected high frequency, said member --being open at both its ends and'said cavity having communication with-said v.open: ends whereby sound waves in the ambient can have access to said conductor side through said opens ends and said cavity.

5. A pressure gradient responsive microphone according to claim 4 wherein said member has a length of the order of a half wave length at said frequency. v

6. A pressure gradientresponsive microphone according to claim 4 characterized by the addition of means for altering the cross sectional area of said cavity.

7. A pressure gradient responsive microphone according to claim 4 characterized by the addition of means for altering the effective length of said closure member.

8. In a pressure gradient responsive microphone, the combination of means providing a magnetic path including an air gap, an elongated conductor mounted for movement in said gap in response to differences in sound wave pressure on opposite sides thereof, and means for enhancing the high frequency response of the microphone, said means comprising a plurality of closure members disposed opposite one side of said conductor in longitudinally spaced relation therealong, each of said members providing a cavity opposite said conductor side which is resonant at substantially a preselected frequency in the high frequency range to be enhanced, each of said members being open at both its ends and the cavity provided thereby having communication with said open ends whereby sound waves in the ambient can have access to said conductor side through said open ends and said cavities.

9. A pressure gradient responsive microphone according to claim 8 wherein each of said cavities is resonant at substantially the same frequency.

10. A pressure gradient responsive microphone according to claim 8 wherein each of said members has a length of the order of half a wave length at the frequency at which maximum compensation is desired.

11. A pressure gradient responsive microphone according to claim 8 wherein each of said closure members is constituted by a section of pipe.

12. A pressure gradient responsive microphone according to claim 8 wherein said members are so arranged that the cavities provided thereby extend longitudinally along said conductor.

13. A pressure gradient responsive microphone according to claim 8 wherein at least one of said members is disposed in proximity to one end of said conductor.

LESLIE J. ANDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,047,777 Hartmann July 14, 1936 2,064,316 Olson et a1. Dec. 15, 1936 2,196,342 Ruttenberg Apr. 19, 1940 2,301,638 Olson 'Nov. 10 1942 

